Computer disk drives store and retrieve data by using a magnetic read/write head positioned over a rotating magnetic data storage disk. The head writes data onto the disk by aligning magnetic poles set in concentric tracks on the disk. The width of the tracks depends on the width of the read/write head used. The narrower the tracks can be made, the more data which can be stored on a given disk. As the size of read/write heads have become progressively smaller in recent years, track widths have decreased. This decrease has allowed for dramatic increases in the recording density and data storage of disks.
In a typical disk drive, the magnetic head is supported and held above the disk surface by an actuator arm. By moving back and forth, the actuator operates to position the head above the disk to read or write data on a desired track. The actuator arm is typically moved by a voice coil motor (VCM) acting as a primary actuator. The problem which has arisen as track widths have decreased, is that the limits of the VCM's positioning precision have begun to be reached. This limited precision has made it increasingly difficult to achieve accurate head positioning. As such, a need has arisen for a means to more precisely position read/write heads.
One approach to achieving finer head positioning has been to employ a secondary actuator to operate together with the primary actuation provided by the VCM. This approach involves placing a microactuator along the length of the actuator arm and configured the arm so the microactuator moves a portion of the arm containing the read/write head. Specifically, the head suspension assembly (HSA) of the actuator arm is divided into a fixed portion and a movable portion. Microactuators are connected between the two portions and positioned to be capable of moving the movable portion of the HSA. Thus, the VCM acts as a coarse actuator and the microactuator as a fine actuator.
Commonly, the microactuators have utilized piezoelectric materials which vary their length when a voltage is applied to them. As shown in FIG. 1, a widely used configuration is an actuator arm 2 which has two piezoelectric actuators 4 mounted between the base plate 6 and the load beam 8. The piezoelectric actuators 4 are positioned about a hinge 7 separating the base plate 6 and the load beam 8.
In this arrangement, the actuators 4 act in a `push-pull` manner to move the load beam 8 relative to the base plate 6. That is, as one actuator 4 constricts and pulls the load beam 8 in the desired direction, the opposing actuator 4 expands to push the load beam 8 in the same direction. At the outboard end of the load beam is mounted a slider 9 which carries a read/write head. As can be in FIG. 1, the actuator arm 2 holds the slider 9 above a disk and by swinging side-to-side, move the slider 9 over the surface of the disk. In turn, the slider 9 positions the read/write head just above the disk surface by flying in the thin airflow layer created by the rotating disk. In so doing, the slider and the head are both kept very close to the disk surface. As the actuator arm 2 is swung back and forth, the movement imparted to the slider 9 is in a plane parallel to the plane of disk's surface. As such, the slider 9 can be moved by the actuator arm 2 for relatively large displacements across the disk and can be moved by the piezoelectric actuators 4 for relatively small displacements.
Some significant disadvantages are inherent with this type of actuator arm. The primary disadvantage is out-of-plane movements which are imparted upon the slider 9 by the movement of the actuators 4. The out-of-plane motions are due to a deformation of the structure of the load beam 8 which occurs when the actuators 4 pull and push on the load beam 8. As a result, as the load beam 8 is moved and deformed by the actuators 4, the slider 9 is both moved across the disk surface and rolled to a certain degree. This rolling may cause one side of the slider to drop closer to the disk surface, which can cause a possible contact with the disk surface. As a result, such contact can damage the data tracks on the disk and decrease the disk drive's overall performance. Clearly, such damage and reduced performance must be avoided.
Another limiting factor to these microactuator designs is that the relatively short displacement stroke of the push-pull piezoelectric actuators arrangement results in limited displacement of the read/write head. As such, these microactuator designs are limited to track following operations and cannot seek data tracks on their own.
Thus, a device is sought which will provide sufficient and precise in-plane motion of the read/write head to augment the displacements from the VCM, allowing for the fine head positioning needed with smaller track widths.